The present invention relates to a measuring device, in particular, an electrochemical sensor (e.g., a detecting element).
Measuring devices are generally known. For example, electrochemical sensors, formed in a so-called finger type of construction, are installed in motor vehicles for determining the oxygen content in the exhaust gases of internal combustion engines. These measuring devices possess one sensor element which is arranged directly at a measuring point and which is tightly secured in a housing.
The sensor element, as is known, has a solid electrolyte arranged between two electrodes. According to the makeup of the sensor element, an additional heating element is provided. To evaluate the sensor signals detected by the sensor element, or to supply the sensor element with the necessary heating voltage, provision is made for electrical connecting lines, which connect the sensor element to a corresponding circuit arrangement.
Since the sensor element, for example in measuring the oxygen content in exhaust gases, is arranged in an area which is acted upon by a relatively high temperature and in which an increased danger of contamination exists, it is known to shield the electrical connecting lines at least in the vicinity of the measuring device. From German Patent No. DG 2805 598, it is known to run the electrical connecting lines in a protective device designated as a hollow lead. This hollow lead, which is also made of an elastic hose, engages with a shoulder (e.g., a collar) of the measuring device housing and forms a sealing seat with the latter. In this connection, a form-locking fit obtains between the protective device and the housing. The conventional sealing seat has the disadvantage that although sufficient protection is afforded against coarse contamination, e.g., from spray water, nevertheless, fine contamination, such as is caused by a mist-like condensation, as occurs precisely in the mounting location of measuring devices in motor vehicles, is not countered by an adequate sealing effectiveness. The mist-like condensation spreads along the sealing surfaces of the protective device and the housing, which form the sealing seat, and in this way penetrates into the interior of the measuring device.
This protective device also has the disadvantage that it does not provide sufficient protection for the electrical connecting lines against external mechanical damage, for example, by being bent sharply and relatively high temperatures arising at the mounting location of the measuring device.
The measuring device of the present invention, is advantageous in that it is simple to construct and offers a high level of sealing protection. An absolute sealing tightness of the sealing seat is obtained by the protective device being connected, with a force-locking and form-locking fit, to the housing via a mounting device, which encompasses the housing and the protective device, to form a sealing seat. As a result of the form-locking and force-locking fit between the protective device and the housing, a barrier is established against contact with even mist-like contaminants, so that the spreading contaminants are prevented from passing through the sealing seat.
In an embodiment of the present invention, provision is made that, at least in certain areas in the area of the sealing seat, the housing has a coating, the coating producing a form and force-locking fit between the housing and the protective device, the coating preferably being a PTFE (polytetrafluoroethylene) coating. In this way, a force-locking fit is achieved between the PTFE coating of the housing and the protective device, which is preferably made of a PTFE material, as a result of the hydrophobic properties of the PTFE material. As a result of the water-repelling effect associated therewith, moisture is prevented from penetrating through the sealing seat into the housing.
Due to its high temperature resistance, the PTFE material is additionally suitable for ensuring a permanent sealing effectiveness for the measuring device even under temperature stressing of the housing.
In a further embodiment of the present invention, provision is made that the coating be a PFA (polyfluoroamide) coating. In this way, the property of the material, namely to develop an adhesive effect starting at a certain temperature, is exploited, in that the sealing seat is heated to this required temperature. As a result of the heating, the PFA coating liquefies and produces a form- and force-locking sealing seat between the protective device and the housing. Provision is preferably made to liquefy the PFA coating only partly, through a controlled heating, in particular by an inductive heating, so that the PFA coating remains stable in form and permits a defined production of the sealing seat. In this way, under the given use conditions of the measuring device, a permanent sealing-off of the interior space of the measuring device is assured in a simple manner at the sealing seat between the protective device and the housing.
In another embodiment of the present invention, provision is made that the protective device be an elastic hollow element, whose shell (e.g.,casing), at least in areas, is structurally formed. In this way, it is advantageously achieved that, as a result of the structured surface, the flexibility of the hollow element is maintained, on the one hand, while a mechanical stabilization is achieved, on the other hand, the stabilization preventing a sharp-edged bending of the hollow lead and thus of the electrical connecting lines which are supported in the hollow lead.
In another embodiment of the present invention, provision is made that the elastic hollow element has on its ends attachment segments, which permit the hollow element to be mounted simply and sealingly on the sensor element, on the one hand, and on a contacting device for the electrical connecting lines, on the other hand. The attachment segments are preferably formed so that they can be permanently connected to the corresponding fitting pieces of the sensor element, or to the contacting device, with the aid of a sleeve-like mounting device. In this way, in an advantageous way, surface contact between the elastic hollow element and the sensor element, or the contacting device, is ensured, assuring that the sealing properties will be lasting, even under the influence of temperature or of mechanical strain.
In addition, in another embodiment of the present invention, provision is made that the elastic hollow element be made of a PTFE material. Since, as is well known, this material has high temperature resistance, and the less the material strength, the less the material is subject to temperature-dependent flow, it is thus ensured that an elastic hollow element having the mounting ends is particularly well suited to the oven purpose. In this way, a lasting, sealing connection, capable of withstanding mechanical stresses, can be created particularly at the attachment segment, which is exposed to the sensor element, i.e., to the relatively high temperatures.
Further advantageous embodiments of the invention are disclosed in the other features indicated in the subordinate claims.